Blender skull joining problem
The warning &quot;Sculpt: Hide, Mask and optimized display disabled.&quot; appears when you have active modifiers on your object (the Boolean Modifier). Then in Sculpt Mode a few things like the Mask and Facesets, etc. don't work.

If you activate Dyntopo it warns you because simple objects like the Cube, Cylinder, etc come with UV maps for texture painting. You find the UV map in the Object Properties Data (the green vert triangle icon on the right side of the screen). You can inspect the map in the UV Editing tab (right next to the main menu). Sculpting destroys the UV map. You can ignore this because you don't want to texture high-poly sculptings anyway. It will make your Blender freeze and crash. If you want to color it use Vertex Painting or retologize it.

You can't sculpt on the cylinder because the Boolean Modifier doesn't work in Sculpt mode and there is no mesh geometry to sculpt on. If you activate dyntopo you will get warning &quot;Not supported in dyntopo&quot; in the modifier tab and the cylinder will vanish if you hide the cylinder in the outline. There are the original cylinder and one temporarily created by the Boolean modifier.


To solve this you need to apply the Boolean modifier. Click the drop-down arrow right next to the blue monitor and camera icons of the modifier. Select 'apply'. It will permanently union (copy) the mesh data. It will not modify the cylinder object. Hide the cylinder object to get rid of the overlapping objects. Last but not least, activate dynoto, choose Draw brush, set strength to a very low value and click on the new cylinder part of the sculpt object. This will add more vertices to the new low poly part so you can use the other brushes without problems.